The present invention relates to a semipermeable polymer layer, which is semipermeable to charge carriers and/or permeable to neutral species, and more particularly to a process of its manufacture in front of or overlying an electrically conductive substrate, and its use and application.
Placing the semipermeable polymer layer as a membrane in front of an electrically conductive substrate is generally known. The determinant membrane behavior to separate charge carriers or/and to be permeable to neutral species is governed by its structure and different driving forces involved in use of the membrane. An incorporation of ion exchanging and ion-conducting components into an individual polymer chain of the membrane allows for selecting a chemical species to be separated from other species; the permeability of the polymer layer further limits passage of the species through the membrane to only one kid of charge carried by the species. The incorporated ion-exchanging and ion-conducting components which are carrying a charge themselves are known as fixed sites. For each charge of the fixed sites, there is a corresponding counterion, which is present in the membrane. The separation capability of the membrane is based on the exchange or counterions with ions of the same polarity present in the solution (adjacent the membrane containing the species to be "separated"). Simultaneously, the exchange of co-ions of the same polarity as the fixed sites is prohibited. Therefore the passage of the co-ions through the membrane is suppressed.
It is desirable that such polymer layers be made pinhole free, chemically inert and show high mechanical stability. Furthermore, the permeation and the exchange procedure or species should be carried out with high selectivity and high velocity.
The application of polymer layers of prior art showing ion exchange and ion conducting properties of a membrane is based on two distinct manufacturing steps:
1. The preparation of an ion exchange resin.
The ion exchange resin is made by chemical polymerization or polycondensation. Such procedures provide a direct incorporation of ionic groups (fixed sites) into the polymer. When chemical mixed polymerization of styrene and divinylbenzene is used, a successively applied chemical introduction of ionic groups is required (F. Helfferich; Ionenaustauscher, Verlag Chemie, Weinheim, 1959).
2. The crocessino of the ion exchange resin to a semipermeable membrane.
The processing is carried out either by dissolving in an appropriate solvent only the ion-exchange resin or a mixture of the ion exchange resin and a binder (referred to as matrix --i.e. polystyrene, polyvinylchloride or polyethylene). The membrane is mechanically attached to the electrically conductive substrate by cast coating procedures, i.e., dip-coating or spring-coating processes. After the solvent evaporation, the polymer remains on the substrate and can subsequently be used as a membrane. Such membranes currently used are made, e.g., of Nafion, a perfluorinated polymer, containing sulfonic groups (Wilson A. d., Prosser H. J. (Eds.); Developments in Ionic Polymers, Elsevier; Vol 2, London, 1983). They are capable of exchanging cations.
The transport of species to be separated through the membrane is a kinetically slow process. Therefore, the permeation of the species through the polymer layer is dependent upon the thickness of the layer. This fact makes the layer thickness an important variable for the permeation rate, which up to now could be exploited only in a limited way by applying casting techniques of prior art. Furthermore, the thickness of membrane layers manufactured by techniques of prior art is strongly dependent on the nature of the electrode surface to be coated. For example, using non pretreated and consequently rough substrate surfaces, the cast layers exhibit a relatively broad thickness range of at least 0.05 mmm to 0.1 mm due to the conditions of their preparation. Consequently, the fabrication of uniform membranes of thicknesses lower than that mentioned above is not feasible. A polished and consequently very smooth surface of the substrate enables the fabrication of uniform layers of thickness of about 200 nm. However, in that case, the thickness of the membrane is dependent upon the nature of the required solvent which is used for the processing of the precursor materials.
With decreasing layer thickness of the polymer the possibility of creating pores and cracks increases. The polymer layer thus loses its unique and advantageous membrane property of separating charges of the permeating species. It is known that the membrane diffusion coefficients of species to be separated by such polymer-coated electrodes are lowered by a magnitude of three to four orders of magnitude when compared to those in the electrolyte solution. Therefore, the hitherto process for making a semipermeable polymer on the electroactive surface of the substrate yields a product with limited application and usefulness. (Espenscheid M. W., Ghatak-Roy A. R., Moore III R. B. et al., J. Chem. Soc., Faraday Trans. 1, 82 (1986) 1051-70.)